In connection with crude oil shortage, there is a search for alternative raw material sources for fuel production. Thus, there is an increasing tendency to develop biological fuels from natural raw materials, such as rapeseed oil, palm oil or the like, which are admixed to the mineral fuels. For this purpose, glycerol esters (triglycerides), which are obtained for instance by pressing and/or extraction from oil-bearing fruits and oilseeds, are converted to saturated, largely unbranched hydrocarbons in a catalytic hydrogenation. During hydrogenation, the glycerol skeleton is converted to propane, so that no valuable substance can be obtained. In addition, double bonds are saturated by hydrogen. The carboxyl groups likewise are saturated by hydrogen or split off as carbon dioxide. Since the crude esters contain impurities which disturb the hydrogenation, an expensive cleaning is necessary in dependence on the raw material and catalyst used.
In FIG. 1, such known process is illustrated schematically. The crude fatty acid glycerol esters are cleaned up in a multistage process, wherein metals and phosphatides are removed by desliming to a content of <20 ppm of phosphatides. In the subsequent bleaching, the content of phosphatides can be reduced to 5 ppm. By thermal deacidification, free fatty acids finally are removed. The fatty acid glycerol esters obtained subsequently are hydrogenated by adding hydrogen, wherein a mixture of saturated hydrocarbons and propane is obtained, which is separated by distillation. The hydrocarbons of higher molecular weight then can further be refined or directly be admixed to the fuels. The further cleaning of propane only is worthwhile in exceptional cases; therefore, it can at best be utilized thermally as heating gas. In general, an economically unsatisfactory yield is obtained, since the value of the saturated hydrocarbons obtained mostly is lower than the value of the feedstocks.
A similar process is known from EP 1 728 844 A1, wherein from biological feedstocks, such as corn, rapeseed, soybean or palm oil, part of the impurities are removed by pretreatment with an acidic ion exchanger resin and a first stream is generated, which subsequently is hydrogenated, in order to obtain a reaction product with a hydrocarbon fraction which includes n-paraffins usable as diesel.
From U.S. Pat. No. 4,992,605 there is also known a process for producing higher-valent hydrocarbons for admixture to diesel fuel, wherein vegetable oils are catalytically hydrogenated, in order to convert the feedstocks to higher-chain unbranched paraffins.
However, these processes also have in common that the vegetable oils must be subjected to an expensive precleaning before hydrogenation, since the catalyst of the succeeding hydrogenation is susceptible to impurities. A variable cleaning quality can lead to a reduced activity of the catalyst, a lower availability of the production plant and therefore higher costs. In addition, crude esters of the feedstocks are in part lost due to cleaning, so that the efficiency of the process is impaired. The glycerol skeleton is hydrogenated to propane, which due to its low value can at best be utilized thermally. In addition, the hydrogenation of the glycerol skeleton involves a higher consumption of hydrogen, whereby the operating costs are increased. Fats of inferior quality cannot be used, so as not to damage the catalyst during hydrogenation.